i'd say voigtlander 21/4, ultron 35/1.7, canon 35/2 or if you want to go extreme, a 15/4.5. its extremely wide and complements well a 50. the 21 would complement well too.
maybe a 35 would be the best bet, just keep both as normals, leaving home with only one according to the place where you'll be shooting at. no changing lenses = no hassle
• right eye
• right index finger
• cameras & lenses
What I've seen around: flickr